The further apart the metals are, the higher their galvanic reactions. A dielectric nipple relies on a plastic insert to isolate different metals. Now, absolutely pure water (doesn't happen in nature), it wouldn't matter much, but the water going through our pipe is not pure, virgin H2O, and thus, has at least some impurities in it, allowing galvanic action and thus corrosion to occur.
The further apart the metals are, the higher their galvanic reactions. A dielectric nipple relies on a plastic insert to isolate different metals. Now, absolutely pure water (doesn't happen in nature), it wouldn't matter much, but the water going through our pipe is not pure, virgin H2O, and thus, has at least some impurities in it, allowing galvanic action and thus corrosion to occur.